Surviving systemic infection requires a rapid and vigorous response by innate immune cells residing in the spleen. Indeed, splenectomized humans and rodents are more likely to succumb to systemic bacterial infections than normal individuals. A prominent component of the splenic immune compartment are invariant natural killer T (iNKT) cells, a distinct lineage of innate lymphocytes that recognize self as well as pathogen-derived lipid antigens. Within hours of activation, iNKT cells rapidly exert an effector response that mediates protection against a number of infectious agents including the gram-positive bacteria Streptococcus pneumoniae, the most commonly identified cause of community-acquired pneumonia. Despite their role in antibacterial immunity, the cellular requirements for and functional implications of splenic iNKT cell activation are largely unknown. Combining a novel method to track the endogenous iNKT cell population in situ with various genetic deletion and cell depletion techniques, the studies in this proposal will reveal the localization and antigen-presenting cells required for elaboration of the iNKT cell effector program following pneumococcal infection. Furthermore, we will determine the relevance of iNKT cell activation in terms of the early protective humoral response to S. pneumoniae. These studies will significantly advance our understanding of the complex interplay between cells of the innate immune system and set the stage for future studies investigating the dynamics of iNKT function during infection and inflammation. PUBLIC HEALTH RELEVANCE: Sepsis resulting from systemic bacterial infections remains a major cause of mortality worldwide. Despite significant progress in vaccines designed to protect against these invasive pathogens, drug-resistant strains continue to emerge and negatively impact the global healthcare system. To more effectively combat these infections, a greater understanding of the immune cell types involved and the mechanisms by which they mediate resistance to infection are needed. The data generated from these studies will significantly advance our knowledge of the earliest events in immunity to systemic bacterial infection and facilitate the design of more effective anti-bacterial therapeutics.